The present invention relates to a greased rolling element bearing, in which elements of the bearing such as the bearing cage or roller ends have been coated with a solid lubricant coating.
Rolling element bearings usually comprise an outer and an inner ring, rolling elements in the bearing space between said rings, and a cage for holding the rolling elements at a predetermined mutual spacing as well as a grease for providing lubrication.
Despite the presence of said grease, during service of such a bearing, the cage and the rolling elements are in sliding contact. This results in friction and wear, which limit the service life of such a sliding contact.
In order to reduce the friction and wear it is known to apply a manganese phosphate-coating in the cage pockets which contain the rolling elements. However, such a manganese phosphate coating is not satisfactory for several reasons. One reason is that these coatings are only useful for "running in" the bearing, e.g. only provide an initial reduction of the friction at the beginning of the service life. After a certain period, the phosphate coating will disappear causing metal to metal contact, a higher running temperature and/or the deterioration of the grease which is located in the bearing. In some cases this can locally lead to "burnt" grease end/or dry running of the bearing. Therefore, some grease lubricated roller bearings with standard manganese phosphate coated steel cages give diminutive tribological protection in the contacts between roller and cage pocket. This will generally lead to the formation of brown bands on rollers and raceways. This is attributed to high sliding contact stress and poor lubrication conditions between roller and cage contacts.
Other factors which adversely influence the condition of the grease in the bearing, the running temperature, and the useful service life of the bearing are the high thermal stressing which occurs between the bearing cage and the rolling elements, and the lack of supply of grease between the components of the bearing.
As a result of these problems, bearings which comprise phosphate-coated cages should be relubricated frequently; if such frequent lubrication is omitted, the useful service life of the bearing is impaired.